The Radiographic Image
Image formation
The X-rays used in medical diagnosis are produced from a small area within the X-ray tube when an exposure is made. They diverge outwards from this area, travel in straight lines, and can be detected by a variety of devices used for medical imaging.
As the X-rays pass through the body, some will be absorbed by the organs and structures within the body whilst others will pass through to the equipment used to form the image.
The term ‘density’ is often used in radiography. It can have different meanings depending on the context. In the diagram below, the X-ray beam enters the body and then encounters various structures. The bone has a high density because it has a relatively high mass per unit volume and consequently will absorb
more X-rays than the adjacent area of lung. The lung contains air, which has a relatively low mass per unit volume and therefore can be said to have a low density. When the beam emerges from the body, there will be more X-rays directly under the area of lung compared with the area directly under the bone.
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The image is then captured using an image-acquisition device. When a relatively large number of X-rays are incident upon the detector (e.g. the area under the lung), the image will appear to be quite dark and may be described as having a high image density. The area under the bone will appear lighter, since fewer X-rays will come into contact with the detector. This area therefore has a lower image density.
When examining an image for disease, the diagnostician may refer to a small focal area of disease as a density within the image. Rather confusingly, this could be of a higher or lower image density compared with the surrounding tissues, depending on the organ or tissue involved,
e.g. a tumour in the lung (higher density)
or bone (lower density, depending on tumour type).
or bone (lower density, depending on tumour type).
In summary, the term ‘density’ can be used in the following ways:
• Patient or physical density: relates to the mass per unit volume of the structures within the patient and their absorption characteristics.
• Image density: the amount of signal detected in the image receptor or, put crudely, ‘blackening’ within the image. If measured on film using a densitometer, this will be optical density.
• In diagnosis: refers to a small defined area of pathology